VEGETATIONAL PATTERNS OF THE ATLANTIC BARRIER ISLANDS
On barrier islands a unique vegetational complex has been established in response to the ocean-dominated environmental complex. Changes in the complex-gradients, largely in response to the barrier island topography, result in a characteristic zonation of vegetational communities. Numerous studies suggest salt-spray deposition as the major environmental factor causing this zonation (Olsson-Seffer 1909a; Oosting 1945, 1954; Oosting and Billings 1942; Boyce 1954; Martin 1959).
The vegetative zones exhibit physiognomic similarities along the entire length of the Atlantic barrier-island chain. Herbaceous plants growing on the primary dunes must be able to withstand high intensities of salt-spray deposition and respond to periodic and regular burial by sand (Wells and Shunk 1938; Kurz 1939; Salisbury 1952). In the more stable and protected areas of the swales and secondary dunes, woody shrubs may dominate. Leeward of the secondary dunes, the maritime forest community usually occurs.
Species of the dunes and swale exhibit several ecological strategies for survival in the barrier-island environment. Often the plants have xeromorphic adaptations of succulence, thick cuticles and epidermal layers, and depressed stomata (Purer 1936; Wells and Shunk 1931; Harshberger 1908, 1909). The features are probably adaptations to the intense winds, salt spray, and high surface soil temperatures, since usually there are not xeric conditions in the subsurface soil. The succulence of leaves, as measured by leaf thickness, decreases with distance from the sea and increases with increased exposure to salt-laden winds (Chrysler 1904; Purer 1934; Boyce 1954). Although the penetration of chloride ions may be the main factor causing hypertrophy in the leaves of coastal plants (Boyce 1951b), other factors such as extremes in wind speed and surface soil temperature may also be important. Compared to plants of flood-plain sites, plants growing on fresh-water dunes exhibit not only increased leaf thickness but also increases in epidermis and cuticle thickness, vessel cross sectional area, vessel wall thickness, and mechanical tissues (Starr 1912).
Not all plants of the dunes and swales have morphological features which may protect them against the osmotic effects of salt spray deposition. Species which exhibit a low tolerance to sea-water spray (Oosting 1945; Martin 1959) fall into two broad categories: (1) Annual plants completing their entire life cycle between major storms; and (2) plants that avoid salt-spray deposition by either a low-growth form or by growing under the protective canopy of more salt-spray-tolerant species.
The maritime forests are frequently dominated by thick-leaved evergreen species. The trees are relatively small with flat-top crowns, the canopy height being limited by the intensity of salt spray deposition. Salt spray also causes the windward edge of the forest to assume an espalier form (Fig. 11) (Boyce 1954). A variety of lianas are usually abundant throughout the forest canopies.
The physiognomic and physiographic zones described above exhibit a remarkable similarity along the whole Atlantic Coast. This is not surprising in light of the relatively constant barrier island environmental complex strongly influenced and moderated by the Atlantic Ocean. However, there are definite changes in species composition along the barrier island chain; some species are found along nearly its entire length, while others are limited to northern, middle, or southern portions (Duncan 1974).
The distributions of coastal dune and forest species reported in the literature for Florida through Massachusetts are presented graphically in Figs. 12 and 13. (Florida: Curtiss 1879; Davis 1942; Kurz 1942; Laessle and Monk 1961; Wallace, et al. 1972. Georgia: Eyles 1939; Johnson et al. 1971; Worthington 1972. South Carolina: Coker 1905; Stalter 1971, 1974; Pinson, 1973; Rayner 1974. North Carolina: Johnson 1900; Kearney 1900, 1901; Wells 1939; Boyce 1954; Bourdeau and Oosting 1959; Brown 1959; Burk 1961, 1962, 1968; Au 1969. Virginia: Egler 1942. Maryland: Chrysler 1910; Shreve 1910; Higgins et al. 1971. Delaware: Snow 1902, 1913. New Jersey: Harshberger 1900, 1911; Chrysler 1930; Small and Martin 1958; Martin 1959; New Jersey Department of Conservation and Economic Development 1969. New York: Taylor 1923; Conrad 1935; Art 1971. Massachusetts: Chrysler 1905; Knowlton 1914; Harper 1921; Fogg 1930; Ogden 1961; Randall 1962; Seacord 1967; Burk 1968.) The spacing between locations in these figures is relative to geographic position along the coastline. Except in the case of Fagus grandifolia (beech), a species with well-known and distinct ecotypes (Braun 1950), the lines representing the range of a species distribution were made by connecting the extreme points of species occurrence and do not imply occurrence on all offshore islands over the ranges shown.
In the Florida Keys, the maritime forest on inundated shores and mud flats is dominated by mangrove species Rhizophora mangle (red mangrove), Avicennia nitida (black mangrove), and Laguncularia racemosa (white mangrove). On sandy substrates in the keys, Pithecolobium guadelupense (black bead), Jacquinia keyensis (Joe-wood), Suriana maritima (bay cedar), and Pinus caribea (Caribbean pine) dominate. On the Atlantic coast of Florida the maritime forest is dominated by Sabal palmetto (cabbage palm), Ilex vomitoria (cassina), Persea borhonia (red bay), and various species of Pinus (pine) and Quercus (oak).
The maritime forest in the Carolinas is dominated by Quercus virginiana (live oak), Juniperus virginiana (red cedar), Ilex opaca, Persea borbonia, and Pinus taeda (loblolly pine) (Fig. 12). Further north, Q. virginiana; Persea borbonia and Pinus taeda are replaced in the community by a variety of Quercus species, Pyrus arbutifolia (red chokecherry), Sassafras albidum, and Pinus rigida (pitch pine). In Eastern Long Island and in Massachusetts, Fagus grandifolia (beech) appears as a major component of the maritime forest.
Except for Quercus virginiana, the tree species comprising the maritime forest are all species that achieve their greatest dominance in inland forests some distance from the coast. However, there is evidence that coastal populations of some species may be genetically different from inland populations (Ledig and Fryer 1972).
Under the maritime forest canopy, there may be such an amelioration of extremes in wind speed, blowing sands, and salt spray that rather nonspecialized herbaceous and shrub flora may occur. There is less of an apparent ecological amplitude of dune and swale species than of maritime forest species. Many dune and swale plants probably can only compete with other species in a barrier-island environment. In general, the species found in the dunes and swale achieve their greatest dominance only in the dune habitat.
The dune and swale community in the southern portion of the barrier island chain is characterized by Uniola paniculata (sea oats), Iva imbricata (marsh elder), Hydrocotyle bonariensis (pennywort), Myrica cerifera (wax myrtle), and Ilex vomitoria (Fig. 13). Northward these species are replaced by Ammophila breviligulata (beach grass), Prunus maritima (beach plum), Hudsonia tomentosa (beach heath), and Lathyrus japonicus (beach pea). Parthenocissus quinquefolia (Virginia creeper), Rhus radicans (poison ivy), and Salsola kali (common saltwort) are found over nearly the entire length of the barrier island system.
Some species which occur on the dunes and swales on certain sections of the Atlantic barrier island chain are found primarily in the salt in other sections. For example Solidago sempervirens (seaside goldenrod) achieves its greatest dominance on dunes and swales on the barrier islands, although it is also found on the margins of salt marshes. However, in Georgia the species is found predominantly in salt marshes (W. H. Duncan pers. comm.). Spartina patens occurs only in salt marshes on northern barrier islands, while in southern sections where dunes appear to be of lower relief and overwash is a dominant geological process (Godfrey and Godfrey 1973, 1974; Dolan et al. 1973) S. patens occurs on low dunes.
The vegetational communities of the barrier islands are composed of the overlapping distributions of individual species. As such, the entire Atlantic barrier-island vegetation can be considered as a vegetational continuum (Curtis 1951) exhibiting a relatively constant physiognomy. Gleason (1926) held that the vegetational composition of a stand is solely the result of plant migration and environmental selection, heterogeneity of vegetation being the result of variance in seed dispersal and irregularities in the environment. It is unlikely that the major changes in species composition along the barrier-island chain are due to limited or irregular dissemination of seeds.
Most of the plant species of the barrier islands have large edible fruits that serve as food sources for a wide variety of bird species (Martin et al. 1951). During the summer I have observed large numbers of catbirds, towhees, robins, and brown thrashers feeding heavily on the fruits of Amelanchier canadensis, Vaccinium corymbosum (highbush blueberry), Prunus serotina (black cherry), Ilex opaca, Sassafras albidum, Juniperus virginiana, Nyssa sylvatica (black gum), Rhus (sumac) species, Smilax rotundifolia (common greenbriar), and a variety of other species in the Sunken Forest. The consumption of fruits and the subsequent dispersal of seeds by birds migrating along the barrier-island system, which is coincident with the Atlantic flyway, is an important factor favoring both northward and southward distribution of species. This is undoubtedly true for Ilex opaca, whose fruits are heavily consumed by migrating birds, particularly robins (Nelson 1965).
Chrysler (1930), after comparing the bird and plant species found on the spit at Sandy Hook, New Jersey, concluded that birds have been the active agents in the transport of seeds to the maritime forest at Sandy Hook. Chrysler further states that the dune and swale flora have probably been derived from the beaches south of Sandy Hook, implying a dispersal by vegetative growth or longshore currents. The sea-water dispersal of seeds and vegetative parts of a variety of coastal dune species has been noted by Van der Pijl (1969), Gemmell et al. (1953), and Salisbury (1952).
The changes in the species composition of barrier-island communities are most likely a response to the gradual changes in edaphic and climatic factors. As stated previously, there are gradual southward changes from siliceous to calcareous sands and gradual increases in temperatures. Although the climatic and edaphic changes of the barrier-island system are not as abrupt as in inland areas of the same geographic range, they appear to be sufficient to cause changes in the species composition of the coastal plant communities.
Last Updated: 21-Oct-2005